MTSO stands for Mobile Telephone Switching Office. The MTSO is the
switching office that connects all of the individual cell towers to the
Central Office (CO).

The MTSO is responsible for monitoring the relative signal strength of
your cellular phone as reported by each of the cell towers, and
switching your conversation to the cell tower which will give you the
best possible reception.

SCM stands for Station Class Mark. The SCM is a 4 bit number which
holds three different pieces of information. Your cellular telephone
transmits this information (and more) to the cell tower. Bit 1 of the
SCM tells the cell tower whether your cellphone uses the older 666
channel cellular system, or the newer 832 channel cellular system. The
expansion to 832 channels occured in 1988. Bit 2 tells the cellular
system whether your cellular telephone is a mobile unit or a voice
activated cellular telephone. Bit's 3 and 4 tell the cell tower what
power your cellular telephone should be transmitting on.

SIDH stands for System Identification for Home System. The SIDH in your
cellular telephone tells the cellular system what area your cellular
service originates from. This is used in roaming (making cellular calls
when in an area not served by your cellular provider).

Every geographical region has two SIDH codes, one for the wireline
carrier and one for the nonwireline carrier. These are the two
companies that are legally allowed to provide cellular telephone service
in that region. The wireline carrier is usually your local telephone
company, while the nonwireline carrier will be another company. The
SIDH for the wireline carrier is always an even number, while the SIDH
for the nonwireline carrier is always an odd number. The wireline
carrier is also known as the Side-B carrier and the non-wireline carrier
is also known as the Side-A carrier.